1. Field of the Invention
The invention relates to welding and refers more specifically to high speed seam welding wherein two or more sheets of metal are joined by a long, continuous, welded seam. The usual purpose of such a weld is to form a leak tight joint between two pieces of metal so that, for example, a fuel tank can be made from two hemispherical pieces of sheet metal welded together along their edges.
2. Description of the Prior Art
Structure used to do resistance seam welding often includes two copper wheels, on opposite sides of metal work pieces to be welded, which are rolled over the work pieces as welding current is applied to heat the metal. Pressure is applied to the wheels to push the heated metal together and form a welded seam. Such action is similar to that involved in making a resistance spot weld except that the wheels turn continuously as the weld is being made.
A commonly used method to improve seam weld quality is to pulse the weld current. In other words, the current used to heat the metal is not applied continuously, but rather it is applied in an on/off pattern. The effect of such current pulsation is to create a series of individual spot welds which overlap to produce a continuous, leak tight seam.
Virtually all larger seam welders use 60 hertz power lines for their power source. Welding current is controlled using primary circuit, phase control, just like with resistance spot welders.
When very fast seam welding speeds are attempted, i.e. speeds of 300 inches per minute and faster, the welding current needs to be pulsed at a rate which is not some integral, multiple of 60 hertz which is the usual power line frequency. Furthermore, for most seam welding applications, the optimum ratio of current on-time to current off-time is on the order of 60% on and 40% off.
Since most large seam welders operate from 60 hertz single phase power lines, such requirements are very hard to meet using conventional techniques. Usually welding speeds must be slowed in order to make good welds with such prior methods and structure.